Control apparatus



March 16 1943. s JONES I 2,313,943

CONTROL APPARATUS Filed llay 4, 1940 INVENTUR. HARRY S. JONES Fatentesi Man recs UNITED STATES PATENT OFFICE CONTROL APPARATUS Harry S. Jones, Philadelphia, Pa., assisnor to The Brown Instrument Company,- Philadelphia, Pa., a corporation of Pennsylvania Application May 4, 1940, Serial No. 333,336

11 Claims. ((01. 158-28) The present invention relates to safety control systems for fuel burners and more particularly to burners having means for distinguishing between normal and abnormal conditions of combustion.

An object of the invention is to'provlde an improved safety control system for a fuel burner which relies in its operation upon the conductivity and rectifying characteristics of a burner flame.

Another object of the invention is to provide a flame responsive safety control system including an electric discharge device, the conductivity of which is changed substantially immediately upon a flame appearing or disappearing by utilizing the conductive and rectifying properties of the Another object of the invention is to provide a flame responsive safety control system which employs a pair of electrodes, one of which may be the burner itself, engaging the flame for measuring the flame conductivity, and wherein means are provided for distinguishing between a con ductive path between said electrodes due to the presence of a flame and other conductive paths between said electrodes, simulating flame conditions,- which may be established, for example, by

carbonization thereof or by accidental relative movement of the electrodes into physical engagemerit.

Another object of the invention is to provide a flame responsive safety control system wherein the possibility of unsafe failures, that is, failures which would permit the fuel valves to remain open with no flame present at the burner is minimized.

Another object ofthe invention is to provide a flame responsive safety control instrument operable from a commercial source of alternating current, wherein failure of any part of the power supply circuit thereof will cause the fuel supply valves to be'closed.

Another object of the invention is to provide a flame responsive safety control system of constant sensitivity, that is, one in which the maximum permissible resistance between the flame electrode and ground is not affected by changes in circuit conditions.

A further object of the invention is to provide a. detector circuit for determining to the value 01' a variable impedance which responds in one suit for determining to the value or a variable impedance having a rectifying characteristic, wherein means is provided to overcome the rectifying action so that thecircuit may be responsive to the impedance value alone.

In combustion control systems heretofore proposed, various means have been employed to determine whether a flame is present at the burner, one such means comprising an electrode extending into the flame of the burner, and so connected to the system as to provide a conductive path of relatively low resistance through the flame to the burner, which is grounded. The difference between the conductivity of this path when a flame is present and its conductivity when the flame is absent is commonly employed to change the bias on the control grid of an electronic valve for controlling a thermal safety switch. Since there is a possibility that a, low resistance path may be established between the electrode and ground through carbonization of the electrode, through accidental contact of the electrode and ground, or through other abnormal conditions simulating combustion, provisions have been made in previously existing devices for preventing the fuel supply from being turned on when such conditions exist.

Since an abnormal condition of this character may occur after the system is in operation, means have been provided which distinguish such cond tions from actual combustion conditions during such operation, and which turn off the fuel feeding means when such abnormal conditions occur.

It is desirable that the sensitivity of such a device be the same for both the starting and operating conditions. That is to say, if a resistance of, for example, megohms between the flame electrode and ground is the maximum which will permit the system to begin normal burner operation, the sensitivity of the device should be such that if during operation, a resistance greater than 150 megohms appears between the electrode and ground, the system will cause the fuel'superal independent secondary windings of a transformer to different parts of the system. It has been found that if certain of these transformer secondary windings should fail while the system is in operation, the system is rendered unresponsive to abnormal combustion conditions. This undesirable feature has been avoided in the device of my present invention.

In systems of the character of the arrangement described wherein an electronic valve controlled by the flame electrode is maintained non-conducting by the presence of the flame, it has been found that failure of the heater filament in said valve might render the system insensitive to abnormal combustion conditions. This also is an undesirable feature and has been avoided in the device of my present invention.

It has been found that the outer parts of a flame contain an excess of negative ions, or electrons, while the inner parts contain an ex cess of positive ions. By the expression outer parts of a flame is meant the region of the flame comprising the outer cone, and similarly the expression inner parts of a flame is intended to mean.the region near the burner comprising the inner cone of the flame. Because of this distribution of the negative and positive ions in a flame, the flame exhibits a rectifying characteristic, that is, it conducts better in one direction than the other. For example, when the electrode is positioned in the outer parts of the flame, it has been found that the conductivity of the flame is greater when the direction of current flow is from the electrode to the burner, or in other words, when a positive potential is applied to the electrode. On the other hand, when the electrode is positioned in the inner part of the flame, the conductivity of the flame will be greater when the current flow is in the opposite direction, that is, when a negative potential is applied to the electrode. Of these two cases, it has been found that the conductivity of the flame is greater when the electrode is positioned in the outer part of the flame, a positive potential being applied to the electrode, and accordingly, the direction of current flow is from the electrode to the burner. It is noted, moreover, that the position of the electrode is considerably less critical when it is placed in the outer parts of the flame than when it is placed in the inner parts thereof in that greater movement of the electrode is permitted without interrupting the system operation, and there is less chance of the electrode accidentally engaging the burner. It is noted also that the rectifying effect is obtained, when the electrode 'is in the outer parts of the flame and positive in potential, with the electrode closely adjacent but actually out of the visible portion of the flame.

This rectifying characteristic of a flame is utilized in the device of my present invention as a means for distinguishing between a conductive path established by the flame and conductive paths which may be established as a result of some abnormal condition. To this end an alternating potential is applied between the flame electrode and the burner and electrical circuit means responsive to the rectifying property of the flame are provided to control the supply of fuel to the burner. Since the position of the electrode in the outer parts of the flame is less critical than it is in the inner parts of the flame and the possibility of direct engagement between the electrode and burner is diminished, as noted hereinbefore, I prefer to position the electrode in the outer parts of the flame.

' Furthermore, in accordance with my invention,

the system will operate with the electrode in any position in the flame or closely adjacent the outer portion thereof. To this end a positive bias is continuously maintained on the electrode, being superimposed on the alternating current applied between the electrode and burner, to overcome the opposing rectifying effect encountered when the electrode engages the inner part of theflame.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and specific objects attained with its use, reference should be had to the accompanying drawing and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

- The single figure of the drawing shows a gas burner i supplied with gas through a conduit 2. The flow of gas to the burner is controlled by an electrically operated or other suitable valve 3. A pilot burner 4 is provided which is conof the flame to the inner parts thereof, whereby trolled by a similar, suitable valve 5. Means are provided for igniting the pilot flame including a pair of electrodes 6 which are connected to the terminals of a secondary winding I of an ignition transformer 8 having a primary winding 9 which may be energized from the alternating current supply lines L and L The fuel valve operating circuit of my control system is controlled by means of a thermostat l 0 which may be located in a room or space to be heated. The thermostat ill may be of any suitable construction and includes a bimetallic element ll connected by means of a conductor 12 to line L, and a contact blade i3 adapted to engage a stationary contact 14 which is connected to line L through a thermal switch I5 and winding [6 of a transformer i'i. i

The thermal switch i5 is preferably of the form disclosed in the Patent 1,958,081 issued to F. S. Dennison May 8, 1934. As shown more or less diagrammatically in the drawing, this switch comprises a stationary arm i8 and a movable arm is biased for movement away from arm ill but normally held in engagement with the latter by means of a bimetallic element 20. Element 20 is rigidly secured at one end to a block 2i and is arranged to be heated by a coil 22 when the latter is energized through a circuit which will later be described. Upon energization of coil 22 for a predetermined period of time, element 20 will be warped sufilciently in the counter-clockwise direction to permit arm l9 acting under spring or other bias to separate from switch arm i8 thereby interrupting the circuit including thermostat iii. The switch l5 will remain locked in this position until manually returned to its normal closed position.

The transformer l1 supplies power for the control system and is a combination step-up and step-down transformer comprising the line voltage primary winding l6, low voltage secondary windings 23 and 24, and a high voltage secondary winding 25. The low voltage secondary winding 23 is connected by conductors 26 and 21 to the heater filaments 28 and 29 of the electronic valves 30 and 3|, respectively. The electronic valve 30 is a triode of any suitable type, and functions in this circuit as a detector. electronic valve 3| is a triode of the type com monly known as a power amplifier valve. The detector 30 includes an anode 32, a control elec- The hereinafter termed the output circuit, which may be traced from the upper terminal of the winding 25, as it appears in the drawing, through a fuse 38, a conductor 39, a resistor 48 and a condenser 4| in parallel, a conductor 42, the anode 32, the cathode 34, and a conductor 43 to the lower terminal of the winding 25 The amplifier 3| is supplied with analternating potential from the transformer secondary winding through a circuit, hereinafter termed the output circuit, whichmay be traced from the lower terminal of the winding 25 through the conductor 43, a winding 44 of a relay 45 in parallel with a condenser 46, a conductor", the anode 35, the cathode 31, the conductor 39, and the fuse 38 to the upper terminal of the winding 25.

The detector is connected to the transformer winding 25 in such a manner that. it may conduct current only during those half cycles in which the-upper terminal of the transformer winding is positive with respect to the lower terminal, while the amplifier 3| is' connected so that it may conduct only during the alternate half cycles, when the lower transformer winding terminal is positive with respect to the upper.

The control electrode 36 of the amplifier 3| is normally connected to cathode ,31 through an input circuit including a protective resistor 48 and resistor and condenser 4| in parallel. When the detector 30 is conductive, a potential drop exists across the resistor 48, a charge is built up on the condenser 4|, and the control electrode 36 is maintained at a negative potential with respect to theca'thode 31. denser 4| and the resistor 48 are so, proportioned that several cycles must elapse before the charge built up on the condenser can leak off through the resistor. Therefore, the control electrode 36 is maintained at a negative potential with respect to the cathode 31 throughout the complete cycle, and the amplifierv 3|, accordingly, will not conduct sufficient current to energize relay winding 44. Conversely, when the detector 30 is non-conductive or carrying little,

. if any, current, the potential drop across the-resistor." will be small, any charge on the condenser 4| will leak ofl after a few cycles, and the control electrode 38 will be at or near the potential of cathode 31. As a result the amplifler 3| will conduct suflicient'current to energize the winding 44 of relay 45. The condenser 45 connected across the terminals of relay winding 44 is provided to smooth out the intermittent current supplied thereto, and maintains the relay energized through the part of the cycle when the amplifier is not conducting, thereby preventing chattering of the relay contacts. It will be apparent that the amplifier3l responds inversely to the condition of detector '48, that is, when the detector conducts a large current, the am-' plifier conducts a small current, and vice versa. For the sake of convenience in describing the operation of this system, the amplifier 3| will be referred to as conductive when it carries sufficient current to operate the relay 45, and as non-conductive when its current is insumcient for that purpose. correspondingly, the detector The con- 30 will'be referred to as conductivewhen its current is large enough to cause the amplifier 30 to be non-conductive, as defined above, and as non-conductive when its current is insufllcient for that purpose.

The control electrode 33 of detector 30 is connected'through a protective resistance 49 and a conductor 58 to an electrode 5| which is insulated from the burner and extends into the pilot flame. The conductor 58 is encased in a shield 52 of any suitable type and the shield is connected to the cathode 34 of detector 38 by a conductor 53 and the conductor 43. The conductor is connected to the conductor 43 through a parallel circuit consisting of a condenser 54 and two resistors 55 and 56 in series. A circuit is provided to shunt the resistor 40 and condenser 4| which may be traced from one terprovided to shunt the resistor minal of the resistor and condenser through a conductor 51A, a switch contact 51, a switch arm 58,.a conductor 58A and the conductor 39 "to the other terminal of the resistor and condenser. The switch arm 58 is controlled by a relay 59 so as to be closed on the contact 51 when the relay is de-energized. Means are also 56 through a switch contact 59A which cooperates with a switch arm 68 controlled by a relay "6| so that the switch isopen when the relay is de-ener gized. The influence of these circuit connections on the control electrodes and 33 of the electronic valves 3| and 30, respectively, and their functional relationship to the system, will be described hereinafter,

The condenser 54 and resistors 55 and 56 are placed in parallel with the distributed capacitance and leakage resistance between the conductor 58 and shield 52. By making the capacitance of condenser 54 large with respect to that of the shielded cable, and the resistance 55 less than the leakage resistance of the shielded cable, the detector circuit is made insensitive to variation in the length of cable which may occur between difi'erent installations of the device.

The relay 59 has a winding 62, and controls,'in addition to the switch arm 58 previously mentioned, a switch arm 63,which cooperates with a switch contact 64 so that the switch is open 15, and arm 14 has a winding 65 and controls, in addition to the switch arm 68 previously mentioned, switch arms 86, 61, and 58, cooperating respectively with contacts 89, 18 and 1|. All switches controlled by the relay 8| are open when the relay is deenergized'. The relay 45 controls switch arms 12, 13, and 14. When the relay 45 is deenergized, switch arm 12 closes a circuit by engaging a contact closes a circuit by engaging a contact 18. When the relay 45 is energized, the arms 12, 13 and 14 move against the contact 11, 18 and 19, respectively.

The winding 62' of relay 59 may be energized of several circuits. All of these circuits may be traced from the upper terminal of winding 24, as it appears in the drawing, through a conductor 80, winding 82, and a conductor 8| to a junction 82. From the junction 82 the circuits may pass through the heater coil 22 of the thermal switch I5 to a junction 83 or, if relays 45 and- 6| are both energized, the heater coil 22 will be shorted, and the'circuits will pass through contact 18, switch arm 13, contact 10 and switch arm 81 to the junction 83. From the junction 83, all the circuits may be traced through a con winding 52 of relay of the thermal switch ductor 84 to a junction 55. From the Junction 85, one circuit may be traced through contact 54, switch arm 53 and a conductor 85 to the lower terminal of the winding 24; a second circuit may be traced through a conductor 81, switch arm 12, contact 15, a conductor 85, contact 59, switch arm 55, a conductor 88, and conductor 55 to the winding 24; while a third circuit may be traced through conductor 81, switch arm I2, contact 11, and conductor 85 to the winding 24.

The winding 55 of relay 5| may be energized through either of two circuits from the transformer secondary winding 24. These may be traced from the upper terminal of winding 24, conductor 80, winding 55 to a junction 95.. From the junction 90, one circuit may be traced through contact 69. switch arm 55, conductor 89, and conductor 85 to the winding 24, while the other circuit may be traced through conductor 88, contact 15, switch arm 12, conductor 81, contact 54, switch arm 53, and conductor 55 to winding 24. I

The circuit for energizing the primary winding 9 of the ignition transformer 5 may be traced from line Ll through a conductor 9|, the primary 9, contact 15, switch arm 14, a conductor 92, contact 1|, switch arm 58, and a conductor 55 to the other line In.

The circuit for energizing the fuel supply valve 5 for the pilot burner may be traced from line L1, through conductor 9|, valve 5, contact 1|, switch arm 58, and conductor 93 to line L2.

The circuit for energizing the main fuel supply valve 3 may be traced from line L1 through conductor 9|, valve 3, contact 19, switch arm 14, conductor 92, contact switch arm 55, and conductor 93 to line L2.

The operation of the system when starting the burner under normal conditions will now be described. It is noted that when the system is deenergized all parts are in the positions shown in the drawing. When the temperature of the room or space to be controlled falls below the value it is desired to maintain, the thermostat l operates to move the switch blade into engagement with the contact l4 resulting in closure of an energizing circuit to the transformer primary winding l5 and thereby energization of the transformer secondary windings.

At this time the control electrode 35 of amplifier 3| will be connected to the cathode 31 through the protective resistor 48, conductor 51a, contact 51, switch arm 58, and conductor 58A. Therefore, the control electrode 55 will he at substantially the same potential as the cathode 31, and the amplifier 3| will be conductive, 'regardless of the condition of detector 50. The winding 44 of relay 45 will then be energized, and the switch arms 12, I3, and 14 will be closed on their upper contacts, as they appear in the drawing.

The closure of the switch arm 12 on contact 11 will complete a series circuit through the 59 and the heater coil 22 I5. This circuit may be traced from the upper terminal of transformer secondary winding 24 through conductor 55, winding 62, conductor 8|, junction 82, heater coil 22, junction 83, conductor 54, junction 85, conductor 81, switch arm 12, contact 11 and conductor 85 to the other terminal of winding 24. The energization of relay 59 will close a holding circuit for itself and heater coil 22, which may be traced along the last described circuit to the junction 85, from where it passes through contact 54, switch arm 55 and conductor 55 to the winding 24. The energization of heater coil 22 being thus maintained, if the cycle of relay operations described below is not completed in a predetermined period of time, the thermal switch |5 will open to deenergize the system and close all fuel valves.

The energization of relay 59 will also cause the separation of switch arm 55 from contact 51, thereby opening the circuit which has been shunting the resistor 45 and condenser 4|, and making amplifier 5| responsive to the condition of detector 55, as previously described. The detector 55 will normally be conductive at this time since the control electrode 55 will be substantially at the potential of the cathode 54, being connected thereto through the condenser 54 and the resistors 55 and 55, there being substantially no difference of potential across those elements. With the detector 55 conductive, the amplifier 5| will become non-conductive, and the relay 45 will be deenergized.

Deenergization of relay 45 will cause an energizing circuit for relay 5| to be completed which may be traced from the upper end of transformer winding 24 through conductor 55, winding 55 of relay 5|, junction 90, conductor 55, contact 15, switch arm I2, conductor 51, junction 55, contact 54, switch arm 55, and conductor 55 to the lower end of winding 24.

The energization of relay 5| effects the closing of a holding circuit for itself, the shunting of resistance 56 in the grid circuit of detector 55, the energization of the ignition transformer primary 9, and the opening of the pilot fuel valve 5. The holding circuit for relay 5| may be traced along the last described circuit to junction 55, from there through contact 59, switch arm 55, conductors 89 and to the other terminal of winding 24. The shunting circuit for resistance 55 is closed by engagement of switch arm 55 with contact 59A. The energizing circuits for the ignition transformer and the pilot fuel valve have been traced previously.

The normal result of opening the pilot fuel valve and energizing the igniter is the occurrence of a flame at the pilot burner. The conductivity of the flame between the electrode 5| and the pilot burner will cause the establishment of a circuit which may be traced from the upper terminal of transformer winding 25 through fuse 88, the conductor 59, resistor 45 and condenser 4| in parallel, a protective resistor 94, which is grounded at 95, the pilot burner 4, which is grounded at 95, the flame conductance, electrode 5|, conductor 50, condenser 54 and resistor 55 in parallel, and conductor 45 to the lower terminal of winding 25. As noted above, resistor 55 is shunted out of this circuit through the closure of switch arm 58 on contact 59A. The function of resistance 55 will be described hereinafter. The condenser 54 and resistor 55 are so proportioned that the current flowing through them leads the transformer voltage, and thereby the anode voltage of valve 90, in phase by an appreciable angle. Consequently, the potential drop across these elements and hence the potential of the control electrode 53 with respect to the cathode 54 will lead the anode voltage of valve 55 by the same angle. The size of that angle is such that the control electrode 55 will become negative with respect to the cathode 54 about one-quarter of a cycle after the detector 55 starts to conduct, thus cutting off the detector output. This reduces the average current through the detector suflicient- 1y so that the amplifier 3| becomes conductive, a previously described, and the relay 45 is again energized.

The re-energization of relay 45 effects the closure of the energizing circuit of the main fuel supply valve 5, and completes a shunt circuit around the heater coil 22 of the thermal switch l5. Both of thesecircuits have been previously described. The circuit of the ignition transformer is simultaneously opened by the lifting of switch arm 14 from contact I6, and the fuel burner then is in full operating condition. This normally continues until the temperature of the room or space being controlled rises above the desired value, when the thermostat l moves the switch blade l3 away from the contact l4, de-energizing the entire system.

If the flame should become extinguished while the system is in operation, the conductive path between the electrode 5| and ground will be removed, and the control eli trode 33'of the detector 30 will tend to assume the potential'of the cathode 34, making the detector 30 conductive. This will cause the amplifier 3| to become nonconductive, thus de-energizing relay 45. This will effect energization of the ignition transformer 8, closing of the main fuel valve 3, and the opening of the shunt around the heater coil 22. If the flame should then reappear before the thermal switch l5 has operated to de-en'ergize the system, normal conditions will be re-established as previously described.v If the flame should fail to reappear, however, the thermal switch |5 will cause de-energization of the entire system.

If the electrode 5| should become accidentally grounded the detector 30 will be made conductive, since the control electrode 33 will then be at substantially the same potential as the anode 32. This will cause the amplifier 3| to become nonconductive, thus de-energizing the relay 45, which will open the shunt circuit around the heater coil 22, and, after a'short time, thethermal switch |5 will open, de-energizing the system.

The amplifier 3| is made unresponsive tothe detector 30 under initial starting conditions through contact 51 and switch arm 58, as described, so that the heater coil 22 will be energized, regardless of the condition of the'electrode 5| or the detector 30 at that time. If 'abnormal condition are present in either of the latter elements, the circuit shunting the heater coil 22 will never be completed, and the switc l5 will consequently open. a

It has been shown that the detector 30 responds to the presence of a low resistance between electrode 5| and ground, by becoming conductive, and that it responds to the presence of a high resistance (absence of flame or open circuit), in the same manner. Between these values of resistance which make the detector 30 conductive, lies the range of permissible variation for the flame resistance.

Since it is possible that a conductive path between the electrode 5| and ground might exist, the resistance of which is higher than the highest normal flame resistance. provision is made for checking the existence of such paths during the-starting operation,'through the use of the resistor 56 in series with the resistor 55. When this resistor is in circuit, any current which may be flowing through the parallel circuit including the condenser 54 and resistors 55 and 56 to the electrode 5| and ground leads the transformer voltage by an angle greater than normal, since under the influence of the heater coil 22. Thus,'. the starting of the burner when such a conducthe proportion of the current flowing through the condenser 54 is increased relatively to that through the resistances 55 and 55. This increases the :phase angle of the control electrode potential with respect to the anode potential and allows a smaller current flowing between the flame electrode 5| and ground to render the detector 30 non-conductive. Thus, the detector 30 is sensitive to a higher resistance between electrode 5| and ground when the resistor 56 is in series with resistor '55. This condition exists during the starting cycle at all times previous to the energization of relay 6|. conductive path exists at the time of starting, the detector 30 will not become conductive at any time. The normal starting cycle would then proceed until the relay 5! and heater coil 22 were energized, whereupon the detector 30, remaining non-conductive, would allow the amplifier 3| to remain conductive and hold the relay 45 energized. No further action would take place in the system until the thermal switch |-5 opened tlve path exists between the electrode 5| and ground is prevented.

The system described is connected so that any short circuit or opening of any transformer winding will cause closing of both of the fuel valves.

If the winding 23 should become open or short circuite'd, the amplifier 3| would lose its emission, relay 45 would be de-energized, and the heater call 22 would be energized, causing the thermal switch I5 to open the transformer primary winding circuit. Also, if the winding 25 should fail relay 45 would be de-energized and effect opening of the transformer primary circuit. If the winding 24 should fail, relay winding would be de-energized, opening the circuits to the fuel valves and igniter, so that the valves would close and the ignition circuit would be deenergized.

The heater filaments 28 and 29 of the detector 30 and amplifier 3| are connected in series, so that failure of either the detector or amplifier heater would cause the amplifier to lose its emission with resultant de-energization of the system as previously described. It is noted that parallel connection of the heater filaments would allow the system to remain in operation upon failure of the detector heater filament, thus permitting an unsafe condition. As illustrated, a fuse 38 is desirably provided in the cathode supply circuit dition of the amplifier circuit. Thus, the critical value of resistance between the flame electrode and ground is the same during both starting and operating conditions.

The pisition of the electrode 5| with respect to the flame is not critical in the device of my present invention. This desirable feature is obtained because the electrode 5| is biased positively with respect to ground. When the detector 30 is conductive, a substantially constant potential drop is maintained across the resistor 4n and condenser 4|, the current flow being such that terminal 40A is negative with respect to terminal 403. This potential drop acts as a battery in a circuit, hereinafter termed the input-circuit of detector 30, which, may be traced from ground Accordingly, if such av 85, through resistor 94, resistor 40 and condenser 4| in parallel, conductor 39, fuse 38, transformer winding 25, conductor 43, condenser in parallel with resistor 55, conductor 50, flame electrode 5|, the flame conductance and grounded burner 4. Tracing this circuit, it will be seen that the electrode 5| is biased positively with respect to ground since it is connected to the positive terminal "B 01' the effective battery, and ground is connected to the negative terminal "1A. The positive bias thus obt' ined is suflicient to overcome the opposing rectifying characteristic of the flame which may be encountered if the electrode 5| is located in the inner parts of the flame, as previously pointed out. The system is therefore sensitive to the presence of a flame, regardless of the electrode position. The constants of the circuit may be proportioned so as to maintain the bias voltage between points 403 and 40A effective to produce its intended function when the current conducted by the detector 30 is too small to prevent conduction by the amplifier 3|, so that the insensitivenessbf the circuit to changes in electrode position with respect to the flame is maintained during all circuit conditions.

If the positive bias were not provided, and if the electrode 5| should be in contact with the lower part of the flame, the rectifying action of the flame would be such that a greater flow of current would occur during the half cycles when the electrode is negative with respect to ground. Such a flow would tend to build up a charge on condenser 54 so as to make the control electrode 33 positive with respect to the cathode 34, which condition might make the detector conductive even if a flame were present.

When the positive bias is provided, however, any charge which may build up on the condenser 54 due to the rectifying action of the flame will be of the opposite polarity, and by rendering the control electrode 33 negative with respect to the cathode 3|, that bias will assist in maintaining the detector 30 non-conductive when a flame is present.

While in accordance with the provisions of the statutes I have illustrated and described preferred embodiments of the present invention, those skilled in the art will understand that changes may be made in the form of my invention as set forth in the appended claims, and that some features of the present invention may sometimes be used with advantage, without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. A safety control system for a fuel burner including in combination, means for producing a flame at said burner, means to control said first mentioned means having a first operating condition in which said first mentioned means is permitted to produce a flame at said burner and having a second operating condition in which said first mentioned means is prevented from producing a flame at said burner, a source of alternating current, an impedance, an electric discharge device having a input circuit including said impedance and an output circuit including said control means and said source of alternating current, the flow of current in said output circuit being adapted to be controlled in accordance with the potential drop across said impedance, said control means being adapted to assume its first condition in response to the flow of current in the output circuit of said discharge device and being adapted to assume its second condition when the current flow in said output circuit is reduced, a pair of electrodes insulated from eachother and positioned in the path of said burner flame, and a second electric discharge device having an input circuit and an output circuit, the output circuit of said second discharge to render the system insensitiveto the direction of the rectifying characteristic of the flame whereby the system is sensitive to the presence or absence of a flame regardless of the position of the electrodes in the flame.

2. The combination of claim 1 in which the circuit connections are suchthat the flow of current through the output circuit of said second discharge device establishes a potential drop across said impedance of the proper polarity to cause said first discharge device to respond inversely to said current flow and the presence of a flame between said flame electrodes operates to maintain said second discharge device substantially onconductive and the absence of a flame or the presence of a path between said electrodes more conductive than a flame operates to render said second discharge device conductive.

3. The combination of claim 1 wherein means are provided to shunt said impedance during initiation of said flame-producing means so as to permit said first mentioned discharge device to become conductive and said control device to assume its first condition irrespective of the current flow in the output circuit of said second discharge device.

4. The combination of claim 1 wherein the first and second discharge devices include heater filaments which are connected in series so that failure of either heater filament will cause the flrst mentioned discharge device to become nonconductive and the control device to assume its second condition.

5. The combination of claim 1 in which the first and second discharge devices include heater filaments, a second source of current, a connection between said second source of current and said heater filaments, and a fuse connected in circuit with said first mentioned source of current to protect the system from damage upon the establishment of a short circuit between said sources of current.

6. The combination of claim 1 in which said firstand second discharge devices include heater filaments connected in series including a transformer having two secondary windings and wherein said alternating source of current is obtained from one of said secondary windings, a connection between the heater filaments of said first and second discharge devices and the other of said transformer secondary windings, and a fuse connected in circuit with said one transformer secondary winding to-protect the system from damage upon the establishment of a short icircuit between said transformer secondary windngs.

7. In combination, a load device, a circuit for operating said load device in respo'nse to the presence of a flame, including a source of altemating current, an impedance, a first electric discharge device having an input circuit including said impedance and an output circuit including said load device and said source of current, a second impedance, a pair of electrodes insulated from each other and positioned so as to be exposed to the flame, a second electric discharge device having an input circuit including said first mentioned impedance, said second impedance and said flame electrodes and having an output circuit including said first mentioned impedance whereby said first mentioned discharge device is responsive to the conductive condition of said second discharge device said first mentioned impedance being so connected in the input circuit of said second discharge device that the potential drop across the first mentioned impedance operates to positively bias one of the flame electrodes as required to render the second mentioned discharge device insensitive to the direction of the rectifying characteristic of the flame whereby said second men tioned discharge device is responsive to the presence or absence of a fiame regardless of the position of the electrodes in the-flame.

8. In combination, a load device, a circuit for operating said load device in accordance with the conductivity of a fiame, comprising a pair of spaced electrodes insulated from each other and exposed to said flame, a source of alternating current, afirst electronic valve having an anode, a cathode, and a control electrode, a connection between one terminal of said source and said cathode, a connection between the opposite terminal of said source and said anode including said load device, an impedance, a connection between said control electrode and said cathode including said impedance, 9. second electronic valve having an anode, a cathode and a control electrode, a connection between said opposite terminal of said source and said last mentioned cathode, a connection between said one terminal of said source and said anode including said impedance whereby the current flow through said first electronic valve is inversely responsive to current flow through said second valve, a second impedance, afirst connection between the control electrode and the cathode of said second valve including said second impedance and a second connection between the control electrode and the anode of said second valve including said flame electrodes, said first connection serving in the absence of a conductive path between said flame electrodes to maintain the control electrode of said second valve at substantially cathode potential thereby rendering said second valve conductive, said second connection serving, in

series, and a-fuse connected in series with said first mentioned source to protect the circuit from damage upon the establishment .of a short circuit between said sources through the heater filament connection.

10. A safety control system for a fuel, burner including in combination, means for producing a flame at said burner, means to control said first mentioned means having a first operating condition in whichsaid first mentioned means is permitted to produce a flame at said burner and having a second operating condition in which said first mentioned means is prevented from producing a flame at said burner, a source of alternating current, an impedance, an electric discharge device having an input circuit including said impedance and an output circuit including said control means and said source of alternating current, the flow of current in said output circuit being adapted to be controlled in accordance with the potential drop across said impedance, said control means being adapted to assume its first conditon in response to the flow of current in the output circuit of said discharge device and being adapted to assume its second condition when the current fiow in said output circuit is reduced, a pair of electrodes insulated from each other and positioned in conductive relation to the path of said burner flame, one of said flame electrodes being positioned in the outer part of the flame and the other electrode being positioned in conductive relation to the inner part of the'flame, and a second electric discharge device having an input circuit and an output circuit, the output circuit of said second discharge device including said impedance and said source of current whereby a potential drop is produced the presence of a path more conductive than a flame between the flame electrodes, to maintain the control electrode of said second valve at a potential such as to render said second valve conductive, and said first and second connections Jointly serving in the presence of a flame between said flame electrodes to establish an alternating potential on the control electrode of said second valve differing in time phase from the potential applied between anode and cathode thereof, thereby rendering said second electronic valve non-conductive.

9. The combination of claim 8 in which said electronic valves each have a heater filament, a second source of current, a connection between said second source and said heater filaments in across said impedance in accordance with the current flow in the output circuit of said second electric discharge device, the input circuit of said second discharge device including said source of current, said flame electrodes and said impedance, the circuit connections being such that the current flow through the output circuit of said second discharge device establishes a potential drop across said impedance of the proper polarity to bias the electrode in the outer part of the flame positively with respectto the other electrode to thereby overcome an adverse rectifying eflfect due to said one electrode being in the inner part of the flame.

11. An electric circuit, to respond in opposite manner to safe and-unsafe values of a variable impedance having a rectifying characteristic and having a range of safe values intermediate high and low values which are unsafe, including a source of alternating current, an electric discharge device having an anode, cathode and a control electrode, a second impedance, a connection including in series said source, said second impedance and the anode and cathode of said electric discharge device to produce a potential drop across said second impedance, a connection to impress the potential drop across said second impedance across said variable impedance so as to oppose said rectifying characteristic, a third impedance, 9. connection between the control electrode and cathode of said valve including said third-impedance, and a connection independent of said second impedance and including said 

